Oxidation catalysts containing manganese and a polyvalent metal, such as copper, cobalt, or silver, have been in commercial use for many years. They are referred to as "Hopcalites". The Hopcalite catalyst containing copper with manganese has been commonly used in gas masks for oxidation of carbon monoxide. Hopcalites may be formed by the coprecipitation of MnO.sub.2 with Cu(OH).sub.2 or other polyvalent metal hydroxide. (See, for example, U.S. Pat. No. 1,345,323 of 1920.) Manganese catalysts containing copper or other polyvalent metal have also been prepared from a solution of the metal salt and potassium permanganate in the presence of hydrogen peroxide. (U.S. Pat. No. 1,971,168 (1934), and U.S. Pat. No. 1,995,353 (1935).) The resulting catalyst is an acid manganate of the copper or other polyvalent metal, such as, for example, Cu(HMnO.sub.3).sub.2. As far as is known, such acid manganate catalysts have not found commercial applications.
Dodman et al in U.S. Pat. No. 3,700,605 (1972) describe a procedure for preparing a manganese catalyst by first forming a manganese oxide precipitate from potassium permanganate in the presence of alkali metal hydroxide. The recovered precipitate is resuspended in water, and an aqueous solution of a salt of a polyvalent metal is introduced into the suspension to react therewith. The patent recommends the use of cobalt, copper, silver or lead ions to replace the alkali metal in the precipitate. This ion exchange reaction is described as being carried out at ambient temperature (e.g. 20.degree. C.). No pH adjustment is disclosed. Dodman et al propose that an inert insoluble substance (e.g. kieselguhr or silica) should be suspended in the potassium permanganate solution as it is converted to the manganese dioxide precipitate, thereby providing a catalyst support.
Dodman et al did not propose the use of "by-product" manganese dioxide as a starting material for catalyst preparation. However, the by-product manganese dioxide resulting from the use of potassium permanganate to oxidize organic substances under alkaline conditions is chemically similar to the manganese dioxide precipitate prepared by Dodman et al. Chemically, they are both alkali metal delta manganese dioxide hydrate. During the course of the experimental work leading to the present invention, it was found that aqueous solutions of cobalt or copper nitrate could be reacted with by-product manganese dioxide under the conditions described by Dodman et al and that the resulting products had catalytic activity for hydrocarbon oxidation. However, for certain applications (viz. paint drying ovens) the cobalt or copper catalysts are not as efficient and do not have as long catalyst life as desirable.
When an attempt was made to react a solution of cerium nitrate with by-product manganese dioxide under the conditions described by Dodman et al, the resulting product still contained an appreciable amount of alkali metal and was not suitable for use as an oxidation catalyst. Further discoveries lead to the development of a process for producing rare earth-manganese oxidation catalysts of superior properties. This catalyst and its method of production are the subject matter of this application.
The present invention provides a means for converting by-product manganese dioxide (which presently has little or no commercial use) to a highly desirable oxidation catalyst. For example, the resulting rare earth-manganese catalysts perform efficiently for the oxidation and deodorization of exhaust gases from paint drying ovens, such as those used in the automobile industry. The catalysts of this invention are therefore believed to be of great value in connection with air pollution control, and to have a wide field of application for oxidation of organic materials in industrial waste gases before they are discharged to the atmosphere.